Macitentan-d4 is intended for use as an internal standard for the quantification of macitentan by GC- or LC-MS. Macitentan is a dual antagonist of the endothelin (ET) receptors type A and B (IC50s = 0.5 and 391 nM in a radioligand binding assay using recombinant ETA and ETB, respectively).1 Macitentan inhibits intracellular Ca2+ increases induced by the endothelin isoform ET-1 in human pulmonary arterial smooth muscle cells (HPASMCs; IC50 = 0.9 nM), contractions of isolated rat aortic rings (pA2 = 7.6 for ETA), and sarafotoxin S6c-induced contractions of isolated rat tracheal rings (pA2 = 5.9 for ETB). Macitentan increases plasma ET-1 concentrations in normotensive rats and decreases mean arterial blood pressure in hypertensive DOCA-salt rats (ED50 = 1 mg/kg). Oral administration (30 mg/kg per day) prevents development of pulmonary hypertension and right ventricle hypertrophy in a rat model of hypertension induced by monocrotaline . It also decreases the number of vascular and tubule-interstitial lesions and amount of glomerular damage in a rat model of diabetes induced by streptozotocin . Formulations containing macitentan have been used for the treatment of pulmonary arterial hypertension.2

|1. Iglarz, M., Binkert, C., Morrison, K., et al. Pharmacology of macitentan, an orally active tissue-targeting dual endothelin receptor antagonist. J. Pharmacol. Exp. Ther. 327(3), 736-745 (2008).|2. Lefaucheur, C., Loupy, A., and Zeevi, A. Complement-binding anti-HLA antibodies and kidney transplantation. N. Engl. J. Med. 370(1), 85-86 (2014).